wilc1000: use flexible-array member instead of zero-length array

Use flexible-array member introduced in C99 instead of zero-length
array. Most of zero-length array was already taken care in previous
patch [1].

[1]. https://patchwork.kernel.org/patch/11394197/



Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1608881703-37060-1-git-send-email-tiantao6@hisilicon.com